In the production of solar cells and solar modules, measurement and rating of the resulting cells and modules at the various stages of production is required for quality control, the proper matching of cells to go into modules, and the rating of the modules at the end of production. Measurement of the solar cells or modules is often done using short pulses of light (Van der Pool, Borden, Wiczer, King, Hyvärinen, Keogh).
Typical pulses can be from Xenon lamps discharging a capacitor, as in a typical photographic flashlamp (Borden, Sinton 1996, Hyvärinen, Sinton 2000), Xenon lamps with a pulse-forming network (Wiczer), flashlamps with control electronics to control the intensity-time profile, or LED flashes. Voltage, current and intensity data can be taken for the entire light pulse (Wiczer, Sinton 1996, Sinton 2000) or for just a point or portion of the light pulse (Borden, Hyvärinen).
Frequently, the acquisition of the current-voltage data for cell or module testing is accomplished by ramping the voltage from short-circuit conditions to open-circuit conditions (or from open-circuit to short-circuit conditions) during a single light pulse while the intensity is close to the intensity of interest in order to obtain the entire current-voltage curve (Wiczer, King, Hyvärinen). This has been shown to result in significant errors in the measurement of some solar cells and modules due to the delayed time response of the solar cell to the voltage ramp (King). This error means that the measured results will not necessarily predict the performance of the solar cell or module under constant illumination, voltage and current conditions, i.e., steady-state conditions.
For example, King indicates that a 20 V/second voltage ramp rate would result in a 1% error in the measured power output of some high-efficiency cells. The method of Hyvärinen, using the example of a linear ramp rate from short circuit to open circuit in 400 μs, results in a ramp rate of about 1700 V/s (per cell in series in a module). This ramp rate is about 85 times too fast for a measurement of the cells described in King to achieve a 1% accuracy relative to the steady state. Some modern commercial cells require ramp rates of less than 5 V/s (per cell in series) in order to have less than 1% of error due to the transient response time of the cell to changing voltage. For a linear voltage ramp from short-circuit to open-circuit voltage as in Wiczer, King and Hyvärinen, this measurement could take 130 ms and is not practical for short light pulses.
Maintaining a constant current during a pulse with varying illumination intensity results in a high rate of voltage ramping and can also result in inaccurate measurements for high-efficiency silicon solar cells (Borden, Ossenbrink). Under constant current conditions, very high voltage ramp rates result as the voltage responds to the changing light intensity during the pulse.
A common solution to these problems is to hold the voltage constant during a light pulse, and to only measure one current-voltage point at each intensity during the pulse (Keogh, Sinton 2005). By holding the voltage constant during the light pulse, the time response of the solar cell or solar module is much better, yielding a more accurate result (Keogh). Using this method, a full curve of current-voltage points at a given intensity is constructed by taking multiple flashes and extracting the relevant data.
This constant voltage method has been shown, however, to have significant error in the case of newer generations of high-efficiency solar cells in cases when the light intensity is not constant during the measurement. Even this best case of maintaining a constant voltage during the light pulse has been shown to have significant transient errors that result in the current-voltage-intensity data being significantly different than the steady-state results that would be obtained under conditions of constant current, voltage, and light. This discrepancy has been shown to be due to changes in the stored charge in the solar cells that can occur even when the terminal voltage at the solar cells or modules is held constant (Sinton 2005). These errors can make this data a very inaccurate predictor of the characteristics of the solar cell or module under constant light conditions such as sunlight. This makes the data from flash testing very unreliable for predicting cell or module performance for these types of solar cells and modules.
These inaccuracies are largest for high-voltage solar cells. As successive generations of solar cells continue to improve, a larger and larger fraction of all solar cells produced will be subject to these measurement errors when measured under pulsed light. Since the majority of solar modules produced today are measured using flashlamp solar simulators, these errors in measurement are becoming a serious issue that requires a good solution.
FIG. 1 illustrates a graph 10 showing the computer-modeled prior art current response of a commercial solar cell to a pulse of light. Graph 10 is a PC1D (Clugston) numerical simulation of the time response of a commercial high-efficiency solar cell. This simulation is with the terminals of the solar cell held at a constant voltage corresponding to the maximum power point of the solar cell (580 mV in this case) at 0.1 W/cm2 of incident power. The instantaneous measured current 20 is delayed, shifted to the right compared to the light intensity 30. The steady-state current 40 that would be measured at each light intensity for a constant light source is shown for comparison. The instantaneous current is lower than the steady state current during the light intensity rise time, and higher than the steady state current during the fall time. Neither is the correct steady-state result for that intensity. There is only one fleeting moment where the curves cross and the instantaneous current accurately predicts the steady-state current.
Even as the incident light drops to zero, current still flows at the constant voltage giving the module an apparently infinite power conversion efficiency after the light has been extinguished. Clearly, this is not predictive of the steady-state power production of a solar cell with no illumination.
Due to the above inaccuracies in the measurement of solar cells and modules, an improved measurement technique is desirable.